In the next-generation power grid called as a smart grid, a smart meter (hereinafter, referred to as an SM) that collects power usage amounts and a server that manages electrical devices are installed to each house or the like. The SM communicates with a meter data management system (hereinafter, referred to as an MDMS) through the power grid. The MDMS collects a power usage amount from the SM disposed in each house or the like at regular time intervals. The information of power usage amounts, which are collected by the MDMS, for example, is used by an energy management system (hereinafter, referred to as an EMS) connected to the power grid. The EMS performs power control such as requesting the SM or a sever arranged in each house within a management target area to suppress the use of power or controlling charging/discharging a storage battery connected to the power grid based on a plurality of power usage amounts collected in the MDMS. In addition, the information of the power usage amounts collected by the MDMS is also used in an accounting server connected to the power grid. The accounting server performs an accounting process for the use of power in each house based on the power usage amount of each house collected by the MDMS. Accordingly, the MDMS stores the information of the power usage amount collected from each SM in a storage server so as to be used later.
However, in a case where the information of power usage amounts stored in the storage server is seen by a supervisor of the storage server or an unauthorized user penetrating into the storage server, the situation of activities and the like in each house can be inferred, which leads to the infringement on privacy. Accordingly, a method is considered in which the privacy is protected by concealing the power usage amounts collected from the SM, and the information is stored in the storage server in a state in which a total amount of the power usage amounts can be calculated so as to be used by the EMS or the accounting server later. In such a case, the data size of the information stored in the storage server is requested to be reduced as much as possible.